The strengthening of glass is useful for a number of applications, including glass slabs used in high power lasers. Strengthening serves to increase the thermal loading capability of the glass, as a means of increasing resistance to thermal shock.
Ion exchange is routinely used by the glass industry as a means of strengthening glass. The result is the introduction of a compressive stress to the glass surface, thereby increasing the capability of the glass to withstand stress without fracturing. The ion exchange, which replaces mobile cations present throughout the glass with larger cations (and frequently, successively larger cations in stages), is implemented by immersion of the glass in a molten salt bath.
One of the problems associated with the use of molten salt baths is the reaction of certain components of the glass with moisture or hydroxyl ion present either on the surface of the glass or dissolved in the salt melt. The reaction product adheres to the polished glass surface and is detrimental to the finish of the glass and its transmission properties. The reaction product may be removed by polishing the surface of the glass following the ion exchange in the salt melt. This however is expensive and time-consuming, and can also result in the removal of part or all of the compressive layer formed by the ion exchange treatment. In phosphate glass, the problem is particularly troublesome, since phosphate glass is in general easily corroded by water or hydroxyl ions in the salt melt.